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| United States Patent |
6,044,126
|
|
Rousseau
, et al.
|
March 28, 2000
|
Process for automatically determining the configuration of a
stereotactic radiosurgery helmet to which can be fitted a plurality of
collimators focused on an irradiation isocenter
Abstract
Process for determining the configuration or configurations [treatment time
(TT.sub.i)/diameter (.phi..sub.i,f) of each collimator] of a helmet for
stereotactic radiosurgery, to which can be fitted an plurality of
collimators focused on an irradiation isocenter, consisting in
automatically optimizing, through iterative dose calculation, the dose
(D.sub.p) received at predetermined optimization points (M.sub.p), by
modifying, in the course of the successive iterations, the treatment time
(TT.sub.i) of at least one shot (i) and the diameter (.phi..sub.i,f) of at
least one collimator (C.sub.f) of at least one shot (i), and by
calculating, at each iteration, an objective function (OF) having as
variables the differences between the calculated dose (D.sub.p) and the
expected dose (ED.sub.p) for each point of optimization (M.sub.p),
iterative calculation of the doses being carried out automatically until
the objective function (OF) satisfies a predetermined optimization
criterion.
| Inventors:
|
Rousseau; Jean (Lille, FR);
Gibon; David (Lille, FR)
|
| Assignee:
|
CH&U de Lille (Lille Cedex, FR);
Centre Oscar Lambret-Centre Regional de Lutte Contre (Lille Cedex, FR)
|
| Appl. No.:
|
102216 |
| Filed:
|
June 22, 1998 |
Foreign Application Priority Data
| Current U.S. Class: |
378/65; 378/148; 600/1 |
| Intern'l Class: |
A61N 005/10 |
| Field of Search: |
378/64,65,68,148,150
600/1
|
References Cited
U.S. Patent Documents
| 4780898 | Oct., 1988 | Sundqvist.
| |
| 5315360 | May., 1994 | Sturm et al.
| |
| 5602892 | Feb., 1997 | Llacer | 378/65.
|
| 5627870 | May., 1997 | Kopecky | 378/65.
|
| 5629967 | May., 1997 | Leksell et al. | 378/65.
|
| 5647663 | Jul., 1997 | Holmes.
| |
| Foreign Patent Documents |
| 0 560 331 A1 | Sep., 1993 | EP.
| |
| 91 01279 | Aug., 1992 | FR.
| |
| WO 97 28845 | Aug., 1997 | WO.
| |
Primary Examiner: Porta; David P.
Attorney, Agent or Firm: Wolf, Greenfield & Sacks, P.C.
Claims
We claim:
1. Process for determining the configuration or configurations of a
stereotactic radiosurgery helmet (5), to which can be fitted a plurality
of collimators (C.sub.f) focused on an irradiation isocenter, each helmet
configuration subsequently corresponding to a shot (i) centered on a
predetermined target point on a given target volume, characterized in
that, on the basis:
of optimization points (M.sub.p) chosen in relation to the target volume,
of an expected irradiation dose (ED.sub.p) at each optimization point
(M.sub.p),
and of one (a single shot) or several (a series of successive shots)
predetermined initial helmet configuration(s),
the dose (D.sub.p) received at each optimization point (M.sub.p) is
automatically optimized, through iterative dose calculation, by modifying,
in the course of the successive iterations, the treatment time (TT.sub.i)
of at least one shot (i), and the diameter (.phi..sub.i,f) of at least one
collimator (C.sub.f) used for at least one shot (i), and by calculating,
at each iteration, an objective function (OF) having as variables the
differences between the calculated dose (D.sub.p) and the expected dose
(ED.sub.p) for each optimization point (M.sub.p), iterative dose
calculation being carried out automatically until the objective function
(OF) satisfies a predetermined optimization criterion.
2. Process according to claim 1, characterized in that, at each new
iteration, the treatment time (TT.sub.i) of a single shot or the diameter
(.phi..sub.i,f) of a single collimator (C.sub.f) used for a single shot
(i) are changed alternately.
3. Process according to claim 2, characterized in that the choice, at each
iteration, either of the shot (i) and of the associated new treatment time
(TT.sub.i), or of the shot (i), of the collimator (C.sub.f) and of the
associated collimator diameter (.phi..sub.i,f) is carried out by random
selection.
4. Process according to claim 1, characterized in that the value of the
objective function (OF.sup.n) at the iteration (n) is given by the
following formula:
##EQU2##
wherein K.sub.p is a weighting factor, which is assigned to each
optimization point M.sub.p, P is the number of optimization points
M.sub.p, and m is positive, and is preferably equal to 2.
5. Process according to claim 1, characterized in that, at each iteration
(n), the Metropolis test is applied to the objective function variation
(.DELTA.OF=OF.sup.n -OF.sup.n-1); and:
if the Metropolis test is favorable, the helmet configurations of the
iteration n are accepted;
then, the optimization criterion is applied; and:
if the optimization criterion is not satisfied, a new iteration (n+1) is
begun starting from the helmet configuration or configurations of the
preceding iteration (n-1) if the Metropolis test is unfavorable, or
starting from the new helmet configuration or configurations determined at
the iteration (n) if the Metropolis test is favorable.
6. Process according to claim 5, characterized in that the control
parameter (T) used in the Metropolis test is constant for a predetermined
number of iterations, termed the range of iterations, and in that, at the
end of each range of iterations, the control parameter (T) used in the
Metropolis test is reduced.
7. Process according to claim 6, characterized in that the reduction of the
control parameter (T) of the Metropolis test is calculated by multiplying
(T) by a factor (.alpha.) of between 0 and 1, and, preferably, equal to
0.9.
8. Process according to claim 1, characterized in that the optimization
criterion is satisfied when the objective function falls below a
predetermined threshold.
9. Process according to claim 1, characterized in that the optimization
criterion is satisfied when the objective function has not decreased over
a range of iterations.
10. Process according to claim 8, characterized in that the helmet
configuration or configurations finally chosen is/are that/those for which
the value of objective function OF has been minimal at the time of
iterative calculation.
Description
FIELD OF THE INVENTION
The present invention relates to the field of stereotactic radiosurgery
and, more precisely, to radiation therapy for small brain lesions by means
of a device making use of a helmet to which can be fitted a plurality of
interchangeable, static collimators focused on one and the same
irradiation isocenter. It relates more especially, to a process for
automatically determining the helmet configuration, or successive helmet
configurations, (the diameter of each collimator and the treatment time)
according to whether the subsequent treatment plan is of the single-target
or multi-target plan.
BACKGROUND OF THE INVENTION
Generally speaking, stereotactic radiosurgery is concerned with radiation
therapy for small intracranial volumes and, for example, for arteriovenous
malformations, or for tumors. It makes use chiefly, at the present time,
of two different techniques that have proved their worth for many years, a
dynamic technique and a static technique.
The dynamic technique involves a single source capable of producing a
narrow beam of ionizing radiation that is mobile in space in relation to
the target volume to be treated.
This first technique makes use of devices mainly constructed on the basis
of linear accelerators, and ionizing radiation is mostly produced by a
source of high-energy photons.
The static technique makes involves a plurality of ionizing beams which,
during treatment, are static in relation to the target volume to be
treated, being sharply collimated and focused on one and the same
irradiation isocenter. The invention falls within the field of the
aforementioned second, static technique, which will now be described in
greater detail.
A stereotactic radiosurgery device using the static technique has already
been described, for example, in French patent application FR 2 672 220 and
in U.S. Pat. No. 4,780,898. Such a device comprises a plurality of sources
of ionizing radiation and, for example, radioactive sources of gamma
radiation, of the .sup.60 Co sources, which are mounted on a hemispherical
device facing a plurality of primary collimators, there being one source
for each primary collimator. A helmet internal to the aforementioned
hemispherical device is fitted with smaller diameter secondary, removable
collimators and enables a plurality of isocentric mini-beams to be
obtained.
Prior to implementing the treatment, a stereotactic frame is placed on the
patient's skull, this frame serving to locate the volume to be treated,
known as the `target volume`, in the mechanical coordinate system of the
treatment device, using an appropriate medical imaging modality
(essentially, X-ray angiography for arteriovenous malformations and
Computed Tomography or Magnetic Resonance Imaging in the case of tumoral
lesions). The same stereotactic frame is used to position the patient's
skull in relation to the helmet of the stereotactic radiosurgery device,
in such a way that the irradiation isocenter of the helmet is in a known
position in relation to the target volume.
To effect a shot at a given point on the target volume, known as the
`target point`, the patient's skull is positioned in such a way that the
irradiation center of the helmet coincides with the target point. When all
the collimators on the helmet are of the same diameter, the spatial
distribution of the dose obtained in a volume assumed to be homogenous is
substantially spherical and centered on the isocenter, the maximum dose
being delivered at the isocenter, and the dose delivered at a distance
from the isocenter equal to the radius of the collimators substantially
amounting to 50% of the maximum dose received by the isocenter. In the
static radiosurgery technique, the treatment of a target volume is thus
comparable with a punching out operation, during which it is attempted to
juxtapose spatially the doses delivered at each shot in such a way as to
cover the target volume in its entirety.
In usual practice, as a function of a predetermined target volume for
treatment, an operator specialized in radiosurgery decides on a treatment
plan, by defining, in a first stage, the number of shots to be effected,
and the target point of each shot, that is to say the point on the target
volume on which, for a given shot, the irradiation center has to be
positioned, and, in a second stage, the configuration of the helmet for
each shot. The configuration of the helmet is to be taken here as
referring to the diameter of the secondary collimators that have to be
mounted on the helmet, and the treatment time for a given shot, that is to
say the duration of the shot. The operator can thus decide that a
single-target plan, that is to say a single shot on a single target point,
is sufficient, if he considers that a single shot will enable a sufficient
dose to be delivered throughout the target volume, or, on the contrary,
decide on a multi-target plan, effecting a series of successive shots on
predetermined target points. The treatment plan and the configuration, or
successive configurations, of the helmet must be chosen not only in order
to cover the entire target volume with the optimum treatment dose, but
also, when the target volume is positioned in the vicinity of a sensitive
area, for example in the vicinity of the optic chiasma, taking care to
ensure that the dose delivered in this sensitive area, and, generally
speaking, outside the target volume, be as small as possible. For this
purpose, the operator takes as his basis a certain number of predetermined
points for which he knows the optimum dose that ought to be delivered at
each of these points these will be, for example, points on the envelope of
the target volume, certain points on the inside of the target volume, and,
if applicable, certain sensitive points placed outside and in the vicinity
of the target volume, and for which the dose must be as small as possible.
Hitherto, and in practice, a specialized operator has had several helmets
with which to define his treatment plan, each helmet comprising a number
of collimators equal to the number of collimators that can be fitted on
the helmet. The collimators for each helmet are of identical or different
diameters from one helmet to another. The configuration of the helmet for
each shot (diameter of the collimators on the helmet and shot duration) is
determined empirically. In order, as a preliminary measure, to validate
his choice of configurations and, if applicable, to change it, the
operator is provided with a software that enables him, on the basis of
each helmet configuration, to effect automatically a three-dimensional
calculation of the dose resulting from the set of shots in the case of a
multi-target plan, or from a single shot in the case of a single-target
plan. The relevance of the configurations for a given treatment plan is
gauged by comparing the three-dimensional distribution of the calculated
dose with the target volume and, as applicable, with the sensitive
volumes.
There are several drawbacks in empirically choosing helmet configurations.
The choice necessarily has to be made by an operator specialized in
radiosurgery, on the basis of his experience. It leads, in practice, to a
three-dimensional dose geometry which is not best suited to the target
volume, which target volume can have any non-spherical contour; as a
result, to cover the target volume in its entirety while, at the same
time, avoiding, as far as possible, irradiating the area of healthy tissue
in the external vicinity of the target volume, the operator has, in
practice, to carry out, almost systematically, a series of several shots,
knowing that he has a limited choice of collimator diameters. Now, each
shot necessitates considerable time for treatment and for positioning the
patient, which leads to substantial operating overheads and discomfort to
the patient. From a financial viewpoint, and to ensure the patient's
comfort, it thus proves necessary to limit the number of shots, and even
to be able to reduce the treatment plan to a single shot.
SUMMARY OF THE INVENTION
The invention aims to provide a process for determining the configuration,
or successive configurations, of a static stereotactic radiosurgery helmet
which overcomes the aforementioned drawbacks. It is based on a process
which, on one hand, is designed to be implemented automatically by a
suitably programmed calculator, and which, as a result, no longer
necessitates any action by a specialized operator, and which, on the other
hand, makes it possible to determine helmet configurations for which the
collimator dimensions are different for one and the same helmet in such a
way that the three-dimensional geometry of the dose delivered is better
suited to the target volume. The main advantage as regards treatment is
that it is possible to reduce the number of shots.
According to the process of the invention, on the basis:
of optimization points (M.sub.p) chosen in relation to the target volume,
of an expected irradiation dose (ED.sub.p) at each optimization point
(M.sub.p),
and of one (a single shot) or several (a series of successive shots)
predetermined initial helmet configurations,
the dose (D.sub.p) received at each optimization point (M.sub.p) is
automatically optimized, through iterative dose calculation, by modifying,
in the course of the successive iterations, the treatment time (TT.sub.i)
of at least one shot (i), and the diameter (.phi..sub.i,f) of at least one
collimator (C.sub.f) used for at least one shot (i), and by calculating,
at each iteration, an objective function (OF) having as variables the
differences between the calculated dose (D.sub.p) and the expected dose
(ED.sub.p) for each point of optimization (M.sub.p), iterative calculation
of the doses being carried out automatically until the objective function
(OF) satisfies a predetermined optimization criterion.
Prior to the invention, it had already been proposed to replace certain
collimators on a helmet by plugs, for the purpose of locally cutting off
one or more isocentric irradiation beams and, by this expedient, of
modeling the isodose contour around the irradiation isocenter. However,
the method of determining the helmet configurations remained empirical,
and was still practiced by a specialized operator, and there has never
been any attempt, to date, to combine, on one and the same helmet,
collimators having non-zero diameters of aperture that were different.
Generally speaking, within the framework of the invention, the notion of a
collimator covers both collimators having a non-zero diameter of aperture
and plugs which, for the purposes of generalization are likened to
collimators the diameter of which is zero.
BRIEF DESCRIPTION OF THE DRAWINGS
Further characteristics of the process according to the invention will
emerge more clearly from a study of the detailed description that follows
of a preferred exemplary form of embodiment, which description is given by
way of a non-limitative example, and with reference to the annexed
drawings, wherein:
FIG. 1 is a diagrammatic representation of a known stereotactic
radiosurgery device, corresponding to the static technique;
FIG. 2 illustrates an example of the geometry of an intracranial target
volume to be treated; and
FIG. 3 is a flow diagram illustrating the main steps in a particular
example of automatic implementation of the process according to the
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1, a stereotactic radiosurgery device for the
treatment of cerebral lesions (arteriovenous malformations or tumors),
corresponding to the static technique, includes a plurality of sources, 1,
of ionizing radiation, which are arranged, statically, on a hemispherical
device, 2, housed inside a shielded cavity, 3, forming a radiation shield,
and a helmet, 5, designed to be received inside hemispherical device 2.
The latter comprises, for each source 1, a primary collimator, 4,
communicating with source 1. Helmet 5 is designed to carry a plurality of
isocentric secondary collimators, C.sub.f, which are interchangeable, and
which, once helmet 5 is housed inside hemispherical device 2, take up
positions respectively facing primary collimators 4. Once helmet 5 is in
position inside hemispherical device 2, sources 1 emit ionizing radiation
in the form of a plurality of mini-beams which are collimated by primary
collimators 4 and secondary collimators C.sub.f, and which are centered on
one and the same irradiation isocenter.
Before helmet 5 is placed on the skull of a patient, a stereotactic frame
(not shown) is fitted to the patient in the usual way, this frame making
it possible, in an initial stage, to locate, in the mechanical reference
system of the radiosurgery device, the intracranial target volume to be
treated, using a suitable medical imaging modality. This can involve, for
example, X-ray angiography, in the case of arteriovenous malformations,
and Computed Tomography or Magnetic Resonance Imaging, in that of tumoral
lesions. In a second stage, with the patient lying on table 6 of the
radiosurgery device, the stereotactic frame is used to position the
patient's skull in relation to helmet 5, in such a way that the
irradiation isocenter is centered on a predetermined target point on the
target volume to be treated. Once this positioning operation has been
completed, the radiosurgery helmet 5 fixed to the patient's skull is
translated in the direction of hemispherical device 2, until helmet 5 is
in position inside hemispherical device 2.
By way of a non-limitative example, the radiosurgery device of FIG. 1 was
constituted by 201 sources of .sup.60 Co, the initial activity of which
was 1.1 10.sup.12 Bq. These sources were arranged on hemispherical device
2 facing 201 157 mm long primary collimators 4. Helmet 5 had a diameter of
40 cm, and was capable of carrying 201 60 mm long cylindrical secondary
collimators C.sub.f. Each secondary collimator C.sub.f could have a
collimating diameter of 4, 8, 14 or 18 mm. Plugs could also be mounted on
helmet 5 in place of the secondary collimators, so as to cut off certain
irradiation beams. In what follows, these plugs will be considered as
constituting secondary collimators of zero diameter. Helmet 5 thus enabled
a maximum of 201 isocentric mini-beams of gamma photons, of 1.17 MeV and
1.33 MeV, emitted by the sources of .sup.60 Co, to be obtained. The
invention is not, however, limited to a stereotactic radiosurgery device
using radioactive sources of gamma radiation of the .sup.60 Co source
type, and can be applied to any type of stereotactic radiosurgery device
which, generally speaking, uses a plurality of isocentric mini-beams of
ionizing radiation. This could include, for example, X-rays, proton,
neutron or electron radiation.
FIG. 2 shows a particular example of three-dimensional geometry of an
intracranial target volume 7 that can be treated using the device of FIG.
1, and located in the mechanical reference system (Ox, Oy, Oz) of the
radiosurgery device. In FIG. 2, volume 7 is represented by a plurality of
points M delimiting the envelope of target volume 7.
Before helmet 5 is placed on the patient, with a view to subsequently
treating a given intracranial target volume, a specialized operator
decides, in a first stage, as to the number of shots that will have to be
effected, and as to the target point of each shot, that is to say the
point on the target volume on which, for a given shot, the irradiation
isocenter of helmet 5 has to be positioned. The method of determining the
positions of the target points can take the form of empirically trying to
ensure optimum coverage of the target volume by a certain number of
spheres (one sphere per shot) of different diameters. Each sphere
substantially corresponds to the volume that would be irradiated by a shot
by means of a helmet 5 on which were mounted only all the secondary
collimators of the same diameter. It should also be noted that this method
of determining the target points can also be carried out automatically,
using suitable software. As this automatic method is known from other
sources, it will not be discussed in detail in the present description. It
would simply be pointed out that this automatic method resides in
optimization of the placing and of the diameters of the spheres by an
algorithm of conjugate gradients making it possible to minimize the
following objective function
##EQU1##
where: N is the number of points M (x.sub.n, y.sub.n, z.sub.n) on the
envelope of the target volume, and d.sub.n,B =(x.sub.n -x.sub.B).sup.2
+(y.sub.n -y.sub.B).sup.2 +(z.sub.n z.sub.B).sup.2 -a.sub.B.sup.2, with
(x.sub.B, y.sub.B, z.sub.B) and a.sub.B representing, respectively, the
co-ordinates of the center and the radius of each sphere.
Once the number of shots and the target points of each shot have been
determined, it is necessary, in a second stage, to determine the
configuration, or successive configurations, of helmet 5, that is to say,
for each shot, the diameters of the secondary collimators C.sub.f that
have to be mounted on helmet 5, and the treatment time, that is to say the
duration of the shot.
Hitherto, this second stage was necessarily carried out empirically by the
specialized operator on the basis of a number of predetermined points for
which the operator knew the optimum dose that would have to be delivered
at each of these points.
The process according to the invention advantageously enables this second
stage to be replaced by a process for automatically determining the
configuration, or successive configurations, of the helmet, based on an
algorithm for optimization through iterative dose calculation, which can
be implemented by suitably programming any known computer. One particular
example of the use of this optimization algorithm will now be described
with reference to the flow diagram in FIG. 3.
The main parameters of the flow diagram in FIG. 3 are as follows
M.sub.p (x.sub.p, y.sub.p, z.sub.p); optimization points
P: number of optimization points
K.sub.p : weighting factor assigned to each point (M.sub.p)
ED.sub.p : expected dose at an optimization point (M.sub.p)
D.sub.p : calculated dose at the optimization point (M.sub.p)
I: number of shots
i: shot number (with i ranging from 1 to I)
TT.sub.i : treatment time for one shot (i)
F: maximum number of beams for a helmet
f: helmet beam number (with f ranging from 1 to F)
.phi..sub.i,f : diameter of the collimator of the beam (f) of the shot (i)
BF.sub.p,i,f : dose beam factor that is assigned to each beam (f) of a shot
(i), for a given optimization point (M.sub.p), and which is calculated
according to the position of the beam (f) in relation to the optimization
point (M.sub.p), taking into account the diameter .phi..sub.i,f of the
collimator corresponding to the beam f and the associated physical data
Kp: Boltzmann's constant
T: control parameter (temperature) of the Metropolis test.
Among the above parameters, the optimization points M.sub.p are points
determined by the specialized operator, on the basis of the target volume
to be treated. These optimization points are constituted, in practice, by
a certain number of points M on the envelope of the target volume to be
treated, to which can be added particular points chosen inside the target
volume, and points located outside the target volume and corresponding to
sensitive volumes for which it is preferable to limit irradiation doses.
For each of these optimization points M.sub.p is defined an expected
optimum irradiation dose ED.sub.p. A weighting factor K.sub.p is assigned
to each optimization point according to the importance that the
specialized operator wishes to ascribe to this point. In other words, the
more the specialized operator thinks it important that the irradiation
dose that will actually be delivered at this point should be the closest
possible to the expected dose, with the helmet configuration, or
successive configurations, which have been previously determined
automatically, the more this coefficient will be important by comparison
with the coefficients assigned to the other optimization points.
Prior to implementing the optimization algorithm according to the flow
diagram in FIG. 3, all of the beam factors BF.sub.p,i,f that are assigned,
for each optimization point M.sub.p, to each beam f of a shot i are
calculated as a function of the position of the associated beam f in
relation to optimization point M.sub.p, that is to say, in particular, as
a function of the position of the target point (the irradiation isocenter
of the helmet) in relation to the corresponding optimization point
M.sub.p, and taking into account the diameter .phi..sub.i,f of the
collimator corresponding to the beam (f) and the associated physical data,
such as, for example, the depth of penetration of the beam at point
M.sub.p. This beam factor quantifies the contribution of beam f in the
irradiation dose that will be received by optimization point M.sub.p. In
the case of a secondary collimator of zero diameter (a plug), the
corresponding beam factor will be zero. Such calculation of the beam
factors is known per se and will not, therefore, be described in detail
herein. For further information on beam factor calculation, readers are
referred to the following publication: Phillips MII: Physical aspects of
stereotactic radiosurgery, New York, Plenum Publishing Corporation, 1993.
These beam factors pre-calculated for each beam f and each optimization
point M.sub.p will be saved in a table of values which will be accessed
automatically during the dose calculation stage, which will make it
possible, at the time of implementing the flow diagram of FIG. 3, to avoid
having to calculate beam factors, which is costly in terms of computing
time.
The different steps of the flow diagram in FIG. 3 will now be discussed.
The flow diagram in FIG. 3 is essentially based on an iterative dose
calculation DP according to the known formula featuring in block 8 of the
flow diagram, and on the evaluation, at each iteration (n), of the value
OF.sup.n of an objective function OF according to the formula given in
block 9 of the said flow diagram. In this formula, the variable m will be
positive which, in a preferred alternative form of embodiment, was fixed
at 2.
Initially, one starts out from one or more predetermined initial helmet
configurations, depending on whether a single-target or multi-target plan
has been decided on. For this purpose, variables TT.sub.i will be
initialized to some initial value. Diameters .phi..sub.i,f will, for
example, be initialized respectively to the values corresponding to the
value closest to the diameter of the spheres found when determining the
target points. At each iteration (n), one begins by changing one of the
helmet configurations, either by changing (step 10a) the treatment time
for one of the shots, or by changing (step 10b) the diameter .phi..sub.i,f
of a collimator. Preferably, at each iteration, steps 10a and step 10b are
carried out alternately. More particularly, step 10a will be carried out
by effecting a random selection of a number of shot i, and by randomly
assigning a new treatment time to the corresponding variable TTi, for
example by adding or removing a predetermined elementary time. Similarly,
step 10b will be carried out by randomly choosing one of the numbers of
shots i, by randomly selecting a number for beams f, and assigning to
variable .phi..sub.i,f a given collimator diameter the value of which is
randomly taken from among the different possible secondary collimator
diameter values that can be used. In the aforementioned example of a
radiosurgery device, given by way of illustration, the diameter values
were 0 (plug), 4, 8, 14 or 18 mm. Once steps 10a or 10b have been carried
out, a calculation is made, in step 11, of the variation in the objective
function OF between the iteration (n) and the preceding iteration (n-1),
and then, two successive tests are applied, namely, respectively, in an
initial stage, steps 12 to 14 and, in a second stage, steps 15 to 16.
In steps 12 to 14, a test, commonly known as the `Metropolis test` is
applied to the objective function variation .DELTA.OF, by calculating a
probability P(.DELTA.OF) according to the formula given in step 12 of the
flow chart in FIG. 3. See, generally, Webb S. Optimization by Simulated
Annealling of three dimensional conformal treatment planning for radiation
fields defined by a multi-leaf collimator. Phys. Med. Biol. 36
1201-26,1991.
The step 13 test consists in comparing the probability P(.DELTA.OF)
calculated in step 12 with a figure of between 0 and 1 and taken at random
at each iteration (n). If the probability P(.DELTA.OF) is less than this
randomly selected figure, the program branches directly to the second
test, known as the `end of optimization test` (step 15), without choosing
the new configuration of the iteration (n) that had been changed by
implementation of steps 10a or 10b. On the other hand, if the probability
P(.DELTA.OF) is greater than or equal to the figure taken at random, the
new helmet configuration of the iteration (n) of steps 10a or 10b is
accepted (step 14) and the program then goes on to step 15. The Metropolis
test illustrated by steps 12 and 13 thus makes it possible, essentially,
to determine whether, at a given iteration (n), the new helmet
configuration that has been randomly determined in step 10a or 10b is
chosen or not chosen.
The second test, in steps 15 and 16, consists in applying an optimization
criterion the object of which is to determine automatically whether it is
necessary to begin a new iterative dose calculation, or if it is, on the
contrary, possible to halt the optimization process. In a precise example
of embodiment, step 15 consists in comparing the value OF.sup.n of the
objective function with a predetermined threshold. When the OF.sup.n value
is below this threshold, the optimization criterion is satisfied, and the
iterative calculation process is halted. On the other hand, when the
OF.sup.n value is above the predetermined threshold, the optimization
criterion is not fulfilled and a new iteration (n+1) is begun. More
especially, knowing that, in the particular example of FIG. 3, the
Metropolis test is applied to the objective function OF, prior to the new
iteration (n+1), it is automatically checked (steps 17, 18, 19) to see
whether it is necessary, as a preliminary, to reduce the control parameter
T of the Metropolis test, by multiplying this parameter T, for example, by
a coefficient .alpha. strictly contained between 0 and 1 and which is
fixed, preferably, at 0.9. For this purpose, the number of iterations
corresponding to a range of iterations during which parameter T has to be
kept constant will have been fixed initially.
In another alternative embodiment, it is also possible to insert an
additional test procedure (not shown) between steps 18 and 19 of the flow
diagram in FIG. 3 which consists in automatically checking whether, over a
given range of iterations, the different successive OF.sup.n values for
the objective function that have been calculated have decreased at least
once. If not, the optimization procedure is automatically halted. If so,
the procedure is continued by going on to step 19, and by beginning a new
iteration.
The optimum helmet configurations finally chosen are those for which the
value of the objective function OF has been minimum at the time of
iterative calculation.
The process according to the invention, whereof a preferred exemplary form
of embodiment has just been described with reference to FIG. 3, can
advantageously be implemented automatically by means of any suitably
programmed computer. At the output, one automatically recovers one
(single-target treatment plan) or several (multi-target treatment plan)
helmet configurations (treatment time TT.sub.i /collimator diameter
.phi..sub.i,f), it being possible for the collimator diameters of a given
helmet to be different and these diameters having been optimized by
calculation so that the real irradiation dose subsequently delivered at
each optimization point M.sub.p is as close as possible to the expected
dose ED.sub.p at each of these points. As a result, the three-dimensional
geometry of the irradiation dose that will be delivered is suited to the
shape of the target volume, which advantageously makes it possible to
reduce the number of shots, by comparison, for example, with treatment
plans for which each helmet used at the time of a shot comprises only
collimators of identical diameters.
On the basis of these helmet configurations automatically determined by
calculation, the specialized operator can then, in a final stage not
forming part of the process according to the invention, perform the
treatment on the patient, fitting to the helmet of the radiosurgery
device, between each shot, the secondary collimators that are of the
appropriate diameter. This placing of the collimators on the helmet
between each shot can also be automated, if necessary, by means of a robot
controlled on the basis of the .phi..sub.i,f data obtained at the output
from the process according to the invention.
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